Hillary Clinton acknowledges a well-wisher during a podium tour at the Democratic national convention in Denver. Photograph: Chris Carlson/AP

Hillary Clinton backed Barack Obama last night to be America's next president in an eagerly-awaited speech intended to end the divisions plaguing the party since the nomination battle ended in June.

"I ask all of you who worked so hard for me, who knocked on doors and made those phone calls, who got in arguments from time to time ... to work as hard for Barack Obama as you did for me," Clinton told a lunch crowd of about 2,500 who also heard from Obama's wife, Michelle.

The stage was then set for her to use her main speech to the Democratic convention in a 75,000-seat football stadium to attack the Bush administration and Obama's Republican rival, John McCain. Clinton's combative stance, in a speech billed as one of the toughest of her political career, contrasted with the tack taken by Michelle Obama and others from the Obama camp on the opening day of the convention, during which they failed to set out detailed policy or to turn on the Republicans.

The Obama camp was resentful that the Clintons dominated the second day of the convention and, with Bill Clinton speaking today, threatened to do the same today.

Hillary Clinton yesterday formally released her supporters from the pledges made in the primary elections to support her, freeing them to unite behind Obama. But her appeal met a mixed reception, with some delegates adamant they would not help get Obama elected or even vote for him. "They are going to go down to the bitter end," said a New York delegate, who had swung from Clinton to Obama.

Clinton's speech came on a day of security alerts in Denver after reports of an alleged racially-motivated plot to kill Obama. It was revealed that three men were in police custody after guns, ammunition and the drug crystal meth were found in a truck that was stopped on the outskirts of Denver. Two high-powered rifles, a telescopic sight, a bulletproof vest, walkie-talkies and wigs were found.

With police on alert for the convention, the materials set off a chain reaction. Police and FBI officials immediately began exploring the prospect of a threat to Obama's life.

"There is a fine line between a credible threat and a threat that is carried out," said Marcus Dudley, a detective with the Aurora police. "It is not up to us to determine whether or not it was a credible threat. We can't sit and wait until folks have done the final analysis. We have a duty to react."

The driver of the truck was named as Tharin Gartrell. Shawn Adolf, who wore a swastika ring and was wanted on drug charges was arrested when police came to the hotel where he was staying in a Denver suburb. He jumped from the sixth floor window landing on a flat roof with a broken ankle. He still ran, but was arrested.

The third man, Nathan Johnson, was arrested at an upmarket hotel in Denver. His statements to police during an eight-hour interview appeared to have raised fears of a threat to Obama. In an interview with a local CBS station while in custody, Johnson accused the other two men of making racist statements about Obama. He said they had spoken about shooting Obama on the day of his speech, saying: "He don't belong in political office or blacks don't belong in political office." Johnson denied being involved in the plot.

Troy Eid, a Colorado federal prosecutor, last night said there was not enough evidence to suggest a plot to harm Obama and although officials were still investigating, and keeping an open mind, he said: "The law recognises a difference between a true threat, one that can be carried out, and the reported racist rantings of drug abusers." But he said he had no explanation why the suspects would have hoarded powerful weapons if not to harm Obama.

"It is a very serious crime to threaten a presidential candidate ... the investigation is still ongoing," Eid said.